More than 85% of HCC cases worldwide retain markers for HBV and HCV, indicating that HBV and HCV are major etiological agents for HCC. In addition to causing chronic inflammation and cell death-regeneration cycles, HBV and HCV encode oncogenic proteins. For example, HBx of HBV and HCV core are oncogenic in transgenic mice, suggesting that these proteins may play a direct role in hepatitis-mediated hepatocarcinogenesis. Because viruses have been invaluable tools for discovering key pathways for human carcinogenesis, our initial study was focusing on the molecular aspect of HBx and HCV core. Our previous results indicate that HBx contains a functional nuclear export signal motif that utilizes the Ran/Crm1 complex, a component essential in nucleocytoplasmic transport of many cellular and viral proteins. We demonstrated that HBx not only uses but also disrupts Ran/Crm1-dependent activities, presumably to prevent a host antiviral response. This finding implicates the Ran/Crm1 complex in the molecular pathogenesis of HBV. Recently, we uncovered a new role of the Ran/Crm1 complex in regulating cellular proteins that control centrosome duplication and mitotic spindle assembly. We revealed nucleophosmin as a novel substrate for Ran/Crm1 to negatively regulate unnecessary centrosome duplication. In addition, we demonstrated a HBV/HBx-dependent activation of RanBP1, a Ran-binding protein that is known to destabilize the Ran/Crm1 complex. Elevated RanBP1 is also observed in HBV-positive liver tissues and in HCC. Increased expression of RanBP1 leads to multipolar spindles and abnormal mitoses. Thus, the combined effects of HBV/HBx contribute to chromosome instability. These findings led us to generate a new hypothesis in which the Ran/Crm1 complex serves as the centrosome duplication checkpoint by providing a 'loading dock' mechanism that controls cellular homeostasis, and the disruption of this complex may result in genomic instability, which may be an early step in viral hepatitis-mediated hepatocarcinogenesis. In addition to HBx, recently we have completed a pilot study by determining HCV core-related gene expression profiles in B lymphocytes. We found that HCV core may evict immunity by selectively suppressing genes involved in antigen presentation. These studies are useful in dissecting viral activities that are essential in hepatocarcinogenesis.